Previous data from our laboratory has shown that the cardiac response to exercise training can be temporally separated into two parts. The first part consists of a change in the coronary vascular response to exercise and an increase in left ventricular enddiastolic volume and has been defined as the partially trained condition (PT). The second part consists of a reduction in heart rate at submaximal work loads, an increase in left ventricular wall thickness and an increase in skeletal muscle aerobic capacity. This will be defined as exercise trained (T). With this temporal separation, it is hypothesized that PT may improve the cardiac response to transient ischemia but that T may not changed the response to transient ischemia. The research will focus on the potential change in the coronary vascular bed and/or a change in the autonomic nervous system control of the heart. The heart and coronary system will be studied in chronically instrumented dogs. The dogs will be instrumented to measure coronary flow (Doppler flow probe and microspheres), aortic pressure, left ventricular pressure, coronary artery pressure, electrocardiogram, and regional myocardial segment length. Transient myocardial ischemia will be produced by inflation of a hydraulic occluder around the coronary artery. The electrical stability of the heart will be determined using the method of the current required to initiate repetitive extrasystoles. The dogs will be exercised on a motor driven treadmill for periods of 4 (PT) or 8 weeks (T). Determination of regional myocardial flow, regional segment length, pressure, and the response to transient ischemia will be compared at these two time intervals. Both subtotal and total occlusion of the coronary vessel will be used to amplify any differences that may be a result of exercise training. The long-term goal of these studies is to determine if daily exercise can be used as a preventive measure to reduce the effect of transient myocardial ischemia. The type and duration of exercise may be critically important in this regard and answers must bed sought to this question. Exercise properly prescribed may serve as a physiological means to reduce the effects of coronary hear disease and/or improve the electrical stability of the heart.